If Evernote's team think this will be so hard, just look at this application be reference:
https://leanote.com/
It able to styling the text with markdown:
The normal rich text editor also able to apply predefined styles to texts:
Please Please Please suport Markdown!!!!!
In Evernote, every time when I need to add heading text, I must adjust the text size and font weight, this action so unconfortable, please think about this.